Just calm down, you still have AGES till the actual HSC exams. Start studying progressively for any subjects that invloves a formula (maths and economics) and practise those now! If you're a reasonably competent individual cramming works best for subjects that requires knowing definitions and concepts, this is purely based on my experience of cramming for economics (but know your formulas for economics now- it will be much easier to skip a chunk of formulas when memorising the definition of unemployment and inflation), business studies and legal studies.
Goodluck, but seriously the only time worth stressing out is one week before the exam. That is when the serious study/cramming starts.